Optical communication systems like networking and telecommunications systems rely upon laser signals for information transmission. Whether via continuous wave or pulsed mode signaling, point-to-point data transfer is achieved through the creation, modulation, amplification, and transmission of laser signals. Data carrying laser signals pass through amplifiers, switches, filters, oscillators, and other optical components that make-up the optical communication system.
To be useful in transferring numerous data packets simultaneously, originating laser signals must have narrow bandwidth. Generally, any suitable laser source produces an output having a relatively narrow bandwidth, a bandwidth in part determined by the lasing medium gain profile and in part by the properties of the cavity within which the lasing medium is disposed. Even narrower bandwidths are desirable. In telecommunication applications, in particular, it is desirable to produce narrower bandwidth laser output signals and narrower bandwidth information carrying laser signals. Narrow bandwidth is important in wavelength division multiplexing (WDM) systems, for example, because each data stream in a WDM system is transmitted at a slightly different wavelength, the data stream bandwidths must be sufficiently narrow to avoid signal contamination. In other words, the bandwidths of adjacent laser signals must be narrow enough so that the laser signals do not overlap spectrally. The ability to set the frequency of a signal with high tolerance is desired in modem-telecommunications applications. It is also desirable to change the operating frequency of a laser to optimize the communication network.
Optical filters are a means of tuning and narrowing the bandwidth of an optical signal. Optical filters may serve many functions in an optical network. For example, they may perform signal processing functions, such as noise filtration and demultiplexing/multiplexing, i.e., where a multi-channel optical signal is separated/combined into its constituent elements. In principle, multiplexers, Mach-Zehnder interferometers, and the like may all be considered as performing optical filter functions.
Optical filters are also used more generally as a means to narrow the bandwidth of an existing laser signal, or energy. For example, high Q value resonators, typically formed of a highly reflective optical cavity have been used to externally tune the frequency of a laser output energy. Such external resonators rely upon the fact that it is possible to stabilize the frequency of a laser by raising the Q of the mechanism that determines the lasing frequency. In effect, a highly tuned filtering action is achieved that allows only a single frequency to be amplified. This can be achieved by either raising the Q of the lasing cavity itself or by coupling a laser with a low Q cavity to an external cavity with a high Q. A few low-noise lasers have been shown in which a high Q micro-cavity, such as a quartz microsphere, emits a stabilized laser signal. While these devices have been used to narrow bandwidths, they have not been used to controllably adjust the peak frequency of a laser output energy.
Some have attempted to adjust the frequency of a laser output energy, but the solutions have proved unsatisfactory. A device for modulating laser frequency has been shown having a portion of the laser emission reflected back into the laser from a moving target. The semiconductor diode lasers used, however, exhibit very large frequency noise components and, therefore, the base laser frequency varies randomly over a large bandwidth. Further, the external cavity used has a low Q due to limited reflectance from the target. So while the peak frequency of the output signal may be changed, the bandwidth profile suffers due to the modulating mechanism. Still others have proposed using Fabry-Perot structures to determine the frequency of a laser output, or optical sensor output, though the solutions here have not resulted in the ability to finely tune the output frequency while maintaining a narrow bandwidth spectral profile of the output across the range of output frequencies.
As the foregoing indicates, known optical filters have been used with laser sources or laser signal propagation media to set the frequency of and narrow the spectral bandwidth of an propagating energy, but the art has not provided a structure or method for producing a continuously tunable output frequency laser that is also characterized by narrow bandwidths such as though desirable in telecommunication networks, like WDM systems. It is therefore desirable to have a structure that controllably sets the output frequency, where such output energy has a narrow bandwidth, thereby allowing the output frequency to be set with finer precision. In addition there is a need for an adjustable filter with narrow bandwidth, high stability and wide tuning range for demultiplexing optical communication frequencies such as those used for WDM systems.